An example of radio tag sensor system disclosed in Patent document 1 is shown in FIG. 1. Radio tag sensor chips 2 which operate with wirelessly transmitted power have respective sensors and store unique identification numbers (IDs). Radio tag sensor chips 2 wirelessly send sensed values of the sensors and the identification numbers through antennas. Reader-writer 41 sends data to and receive data from radio tag sensor chips 2 through antenna 31, and wirelessly supplies electric power to radio tag sensor chips 2. A plurality of radio tag sensor chips 2 are mounted on sensitive surface skin 1 of a robot or the like, and single reader-writer 41 and antenna 31 read the sensed values from radio tag sensor chips 2.
Since radio tag sensor chips 2 require no wiring, they are advantageous in that many chips can be placed with a high density and their costs is lowered, because they are free of the problem of reduced reliability which would otherwise be caused by wire disconnections.
The radio tag sensor system disclosed in Patent document 1 can only use radio tag sensor chips within a range wherein they can be wirelessly supplied with electric power from a single reader-writer and an antenna. Therefore, the radio tag sensor system is problematic in that its sensed value collection region where in can collect the sensed values is small.
In order to achieve a wider sensed value collection region, it is proposed that a radio tag sensor system have a plurality of reader-writers and have a reader-writer management terminal to manage the reader-writers, as with a system disclosed in Patent document 2.
Specifically, in the system disclosed in Patent document 2, the reader-writer management terminal and the reader-writers have respective IP addresses, and communicate with each other using the IP addresses. Each of the reader-writers reads and writes the tag information of an RFID tag that is positioned in its own radio communication area, and sends the tag information read from the RFID tag to the reader-writer management terminal according to a command that is stored in an IP packet sent from the reader-writer management terminal by way of unicasting.
The reader-writer and the RFID tag may exchange signals according to a system disclosed in Patent document 3.
Specifically, the system disclosed in Patent document 3 is a book searching system wherein a reader-writer identifies a unique ID of an RFID tag applied to a book to be searched for, and radiates a unique ID indication signal that is indicative of the identified unique ID to a plurality of RFID tags. Each of the RFID tags compares its own stored a unique ID with the unique ID indicated by the unique ID indication signal received from the reader-writer, and, if the compared unique IDs match with each other, returns a unique ID reply signal to the reader-writer.